There are a variety of problems faced by manufacturers who use fluoropolymers for foamed insulation, (e.g. insulated conductor manufacturers), that are not resolved by currently available conductor insulation materials. One such problem for these manufacturers is that the extrusion/melt draw-down process is carried out under a variety of conditions and parameters, resulting in variation of the physical and electrical characteristics of the foamed fluoropolymer for the manufacturers. The manufacturers seek to minimize the variation of the foamed fluoropolymer characteristics.
Another concern of the manufacturers is the economics of extruding the fluoropolymer for a variety of uses. Faced with the onset of insulation quality (spark and/or lump) problems, and at least the uncertainty of changing draw down ratio (DDR), operating temperatures, and cone length, insulated conductor manufacturers typically reduce line speed until the desired quality of the insulated conductor is achieved, which results in a loss of productivity.
The following disclosures may be relevant to various aspects of the present invention and may be briefly summarized as follows:
U.S. Pat. No. 4,764,538 to Buckmaster et al. discloses synergistic combinations of boron nitride (BN) and certain inorganic salts which provide enhanced foam nucleation in fluoropolymers.
U.S. Pat. No. 4,877,815 to Buckmaster et al. discloses a class of sulfonic and phosphonic acids, and salts of the acids which give very efficient foam cell nucleation in a wide variety of thermoplastic materials at low concentrations. Additionally, these acids and salts are beneficially used in minor amounts in conjunction with boron nitride and calcium tetraborate together, i.e. a combination covered by U.S. Pat. No. 4,764,538. The above-mentioned patents do not disclose compositions that can be extruded at high speeds desired by manufacturers while still providing the insulation crush resistance and electrical performance desired by manufacturers such as insulated conductor manufacturers.
It has been found that the temperature of the molten polymer in extrusion must be closely controlled to achieve excellent performance. Loss of control results in unacceptably high incidences of insulation faults such as sparks (points at which the polymer inadequately coats the conductor) and lumps (regions of irregular geometry of the insulation). It has further been found that lot-to-lot variations in the fluoropolymer melt flow rate can upset the close control of extrusion and require time-consuming and wasteful adjustments, during which time unsaleable product is made. Reduction in fluoropolymer melt flow rate variation would impose significant economic penalties. Further polymer improvement is needed to permit high speed extrusion, particularly for extrusion of fluoropolymer insulation with few or no sparks or lumps over a broader temperature range than is possible in the prior art.
It is desirable to have a foamable fluoropolymer composition that can be extruded at higher speeds than presently possible with commercial polymer without risking quality, loss of productivity or desirable characteristics such as the electrical properties of an insulated conductor. It is also desirable to provide an extruded foamed fluoropolymer cable which reduces variation in the processing and electrical performance of the cable. It is further desirable to provide an extrusion process that reduces the amount of polymer material required for the extruded product due to the use of a foamable composition.